The present invention relates to vehicle transmissions, especially for heavy, on- and off-road vehicles, and more particularly to dual- and multi-clutch transmissions with a central synchronizing unit that facilitates the preselection of gears.
Dual clutch transmissions are a cross-breed between conventional stepped transmissions, with power interruption at gear shifts, and powershifting, without power interruption, planetary transmissions. In principle, a dual clutch transmission has two input shafts, each connectable to a friction clutch and to the output of the engine. Functionally, this is equivalent to having two conventional transmissions in parallel. i.e., two parallel sub-transmissions, and using one at a time for power transfer. The sub-transmission that is not used, idling, for the time being, can have a gear engaged and prepared, preselected, for a subsequent shift. This shift is carried out by simultaneously disengaging the friction clutch of the previously used sub-transmission and engaging the friction clutch of the previously idling sub-transmission.
When properly designed, dual clutch transmissions have the potential of providing powershifts at a reasonable production cost and low power losses. This is due to the fact that the rotating parts, i.e., gearwheels, shafts and tooth clutches, are similar to those in conventional stepped transmissions. This, furthermore, enables the use of the same production equipment. So, it makes sense to produce dual clutch transmissions in the same facilities as used for conventional stepped transmissions.
Dual clutch transmissions often have two separate countershafts, one connected to each input shaft. One example is U.S. Pat. No. 4,876,907. These countershafts make the transmission considerably wider than a conventional stepped transmission. That may lead to difficulties in installing the transmission into the vehicle. However, some dual clutch transmission designs have only one countershaft, e.g., as in DE923402 and DE3131156A1. Loose gearwheels are arranged rotatably thereon and can be rotationally connected to each other and to the countershaft by mechanical tooth clutches. In a way, this could be seen as if the second countershaft is arranged coaxial to the first one. The result is a very compact powershiftable dual clutch transmission that is not wider than a corresponding conventional stepped transmission.
Normally, in a dual clutch transmission, gears are preselected in the presently idling sub-transmission by engaging and disengaging tooth clutches. For a smooth and durable operation, this requires that the parts to be engaged by a tooth clutch are synchronized, i.e., that they have fairly equal rotational speed. If not, the clutch teeth would clash, resulting in worn, or broken, teeth and noise. So, different kinds of devices and arrangements are used for synchronizing parts to be engaged. This is also the case for conventional stepped transmissions that have a power interruption at each gear shift. There is, however, one important difference. At a power interruption, the engine speed can be controlled in order to synchronize parts to be engaged. This is a procedure used in automatic mechanically engaged transmissions (AMTs) that are common in heavy trucks and buses. In a dual clutch transmission without power interruption, this is not possible. Instead, some synchronizing devices are required.
A concept, readily known by a person skilled in the art, is to use synchronizers, i.e., every tooth clutch is equipped with synchronizing means, as in US2008/0188342A1. That would imply increased costs and power losses, though.
In FR1445735, hydraulic pumps and valves are used to change the speeds of gearwheels and shafts for synchronizing. This is a costly and complex design. That is also the case for DE10217746A1, having, bulky arrangements of hydraulics and centrifugal weights.
Electric motors could be used for synchronizing the idling sub-transmission. There could be one motor acting on each sub-transmission, or a single motor that can selectively be drivingly connected to any of the sub-transmissions by means of gearwheels and tooth clutches. Two examples are DE19850549A1 and DE19950679A1. As an alternative, a single motor could be kinematically connected to both sub-transmissions via a differential, e.g., as DE19940288C1, EP0845618B1, DE10037134A1, and WO2007/042109A1. However, the peak power required for synchronizing is fairly high. Thus, designs like these would only make sense in hybrid electric powertrains. Otherwise, they would be too bulky, heavy and expensive.
Mechanical frictional clutches can be a powerful, compact and cost-efficient solution for synchronizing in dual clutch transmissions. It can be noted that, in principle, relative to the active sub-transmission, the speed of the idling sub-transmission needs to be either decreased, for preselection before an up-shift, or increased, before a down-shift. This can be embodied by a brake and some kind of a speed-up device, respectively, as in DE3739898A1.
Increasing and decreasing the speed of the idling sub-transmission can be performed by using gearwheels of the lowest and highest gears, respectively.
Practically, the tooth clutches for these gears are each arranged in parallel with a frictional clutch. DE10232836A1 shows a dual clutch transmission where sub-transmissions 14 and 17 have synchronizing clutches; 18 and 20 connect gearwheels 1 and 2 of the lowest gears, 19 and 21 connect gearwheels 9 and 10 of the highest gears. Hence, no additional gearwheels are needed, but four synchronizing clutches, each with control means, are required. That leads to high costs and power losses.
Basically, only two synchronizing clutches are required; one that will make the speed of the first sub-transmission larger than that of the second sub-transmission, and one that will make the speed of the first sub-transmission less than that of the second. This will work when the first sub-transmission is idling and the second is active, as well as when the first sub-transmission is active and the second is idling. Such a device can be referred to as a central synchronizing unit. In WO03/083325A1 this is embodied with two planetary gear trains 30 and 40 that are controlled by a frictional device 50. The design in GB2110324A uses two sets of gearwheels, 33-39-35 and 37-40-36, and frictional clutches, 35 and 38. This will reduce power losses and simplify the control means, but the costs will be large with two planetary gear trains or six gearwheels just for synchronizing.
A central synchronizing unit in a dual clutch transmission can be simplified further. This requires, however, that shifts without power interruption are performed between consecutive gears, only. Furthermore, the speed ratio steps between consecutive gears should be fairly equal. For heavy on- and off road vehicles, in combination with a range section (e.g., as in US2008/0188342A1), this is plausible. In U.S. Pat. No. 4,876,907 a central synchronizing unit 30 makes use of a gearwheel 13 that is used for power transfer. Then, only three additional gearwheels, 32, 33 and 39 are required for the synchronizing function. These gearwheels only need to carry the loads at synchronizing, and can be considerably narrower than the power transferring gearwheels. The central synchronizing unit can, hence, be made very compact, especially in axial extension. In U.S. Pat. No. 5,974,905 it is shown how these additional gearwheels can be used for power transfer, too, giving additional gears. This requires some axial space, though.
The technical journal article Franke, R.: “Das automatische Doppelkupplungsgetriebe fur sorbs oder acht lastfrei, ohne Antriebsunterbrechung und ohne Verspannung schaltbare Gauge”, ATZ Automobiltechnische Zeitschrift (ISSN 0001-2785 10810), vol 101 (1999). No. 5, p. 350-357 presents dual clutch transmissions with a central synchronizing unit that requires only two additional gearwheels. Two frictional plate clutches are used as synchronizing clutches. These clutches are arranged axially overlapping, one on each of two parallel shafts. This makes the central synchronizing unit compact in axial direction, but the control means for the plate clutches, on parallel shafts, will become complex and costly.
Thus, a technical problem addressed by the present invention is therefore how to provide synchronizing means that are more i) compact, ii) powerful, and iii) cost-efficient, and iv) have low power losses when not actuated.
Thus, it is desirable to solve the above problem to provide an improved synchronizing means.
According to an aspect of the present invention, a multi-clutch transmission for a motor vehicle is provided with at least one prime mover, said multi-clutch transmission comprises (includes, but is not necessarily limited to) frictional clutches drivingly connected to said prime mover, an output shaft, and a main transmission comprising input shafts connected to said frictional clutches, a countershaft parallel to at least one of said input shafts, gearwheels, tooth clutches, and a central synchronizing unit, where by selective engagement of said frictional clutches and tooth clutches, different speed ratios between said prime mover and said output shaft can be established, and by selective actuation of said central synchronizing unit, engagement of said tooth clutches can be facilitated, said central synchronizing unit comprising an axially movable synchronizing member that is arranged on and rotationally locked with said countershaft, in actuated states of said central synchronizing unit, said synchronizing member is displaced axially into engagement with mating portions of two of said gearwheels that are rotatably arranged on said countershaft. The invention is characterized in that said synchronizing member comprises an internal and an external conical friction surface that are axially overlapping each other, and that said mating portions on said two of said gearwheels are mating conical surfaces.
According to one embodiment of the device according to the invention said synchronizing member is a countershaft synchronizing double cone.
According to another embodiment of the device according to the invention on at least one of said two of said gearwheels, the gear teeth are overlapping said mating conical surface.
According to one embodiment of the device according to the invention on both of said two of said gearwheels, the gear teeth are overlapping said mating conical surface.
According, to a further embodiment of the device according to the invention at least on of said two of said gearwheels is used for power transfer between said prime mover and said output shaft.
According to another embodiment of the device according to the invention said synchronizing member after actuation is recoiled to, and held at, a neutral axial position by a resilient neutral device.
According to one embodiment of the device according to the invention said neutral resilient device is a preloaded spring.
According to a further embodiment of the device according to the invention said synchronizing member is actuated by an actuator via a control rod that is arranged in a coaxial bore of said countershaft.
According to one embodiment of the device according, to the invention said actuator is non-rotating, said control rod is rotating with said counter shaft, and there is an axial connecting device between said actuator and said control rod.
According to another embodiment of the device according to the invention said control rod and said synchronizing member are connected axially by a lateral member through a radial opening in said countershaft.
According to one embodiment of the device according to the invention said radial opening and said lateral member provide a rotational connection between said countershaft and said synchronizing member.
According to a further embodiment of the device according to the invention said lateral member is a synchronizing transfer pin.